Twisting is an important step of short fiber spinning. In this process, the yarns are twisted and transformed to attain sufficient strength, wear resistance and smoothness. However, as a negative effect, a large amount of residual torque or twist liveliness is also brought about in the yarns simultaneously. Such twist liveliness of the yarns significantly influences the quality of the resulting products. For example, if yarns with twist liveliness are used for knitting, loops of the fabric will lose their balance because of the residual torque in the yarns. In order to attain the natural structure with the minimum energy condition, the loops tend to rotate to release the internal torsion stress. As a result, one end of the loops will tilt and protrude from the fabric surface, while the other end will stay inside the fabric. Such deformation of the loops will increase the spirality of the fabric, i.e., a deformation similar to the rib effect, which should be prevented to the greatest extent possible. Thus, the balancing of torque inside the yarns is particularly important.
Staple yarns are made from a large quantity of fibers bonded by the friction between the fibers. Hence, the residual torque of the yarns or the spirality of the fabric is mainly affected by the friction-related characteristics of the fibers, such as the type and cross-sectional shape of the fibers, the polymerizing manner of the fibers and the internal structure of the yarns, etc.
First of all, different types of fibers have a different modulus and cross sectional shape, thus leading to different degree of stress in the yarns. In cotton/polyester blended yarns, increasing the ratio of polyester will enhance the twist liveliness of rotor and ring yarns, but heat setting can improve the spirality of the resultant fabrics.
This is because polyester has a higher modulus than cotton, and said two types of fiber have different cross sectional shapes.
Next, different yarn structures have a different distribution of stress. Experimental results, such as Barella and Manich in the Textile Research Journal, Vol. 59, No. 12, 1989, Lord and Mohamed in the Textile Research Journal, Vol. 44, No. 7, 1974 and Sengupta, and Sreenivasa in the Textile Research Journal, Vol. 64, No 10, 1994 showed that, friction spun yarns (DREF-II) have the largest residual torque and trend of deformation in the priority sequence followed by ring yarns, rotor yarns and air-jet yarns. It is generally agreed that singles ring yarns are composed of a plurality of uniformly enveloped concentric helical threads, while fiber migration is a secondary feature. Hence, when the ring yarns are reverse-twisted, their strength will gradually decreases to zero, by then the yarns will be all dispersed. In relation to ring yarns, unconventional spinning systems produce yarns with core-sheath structures, such as rotor spinning yarn, air jet spinning yarn and friction spinning yarns. The packing density of said yarns is uneven and mainly characterized by partial entanglement and entrapment of the fibers.
In addition, many factors can affect the degree of movement freedom of the loops of the fabric and also the final spirality of the fabric. Said factors include fabric structure, parameters of the knitting machine, and the fabric relaxation and fabric setting due to finishing. All the aforesaid factors affecting the spirality of fabric were reported in detail by Lau and Tao in the Textile Asia, Vol. XXVI, No. 8, 1995. As with other materials, the residual torque of the yarns can be reduced or eliminated using different methods. In the past several decades, a variety of torque balancing methods have been developed. According to the basic theory, they can generally be split into two categories: permanent processing methods and physical torque balancing methods.
Permanent setting methods mainly accomplish the purpose of releasing residual torque by transforming the elastic torsional deformation into plastic deformation. The method mainly relates to a variety of setting techniques for material, such as thermal setting, chemical processing and wet setting etc. In the Textile Research Journal, Vol. 59, No. 6, 1989, Araujo and Smith have proved that for air-jet and rotor yarns, the heat setting of singles cotton/polyester blended yarns can effectively reduce the residual torque of the yarn. However, in relation to natural fibers such as cotton or wool, permanent setting is more complicated. It may involve steaming, hot water and chemical processing (such as mercerization in the case of cotton yarns and treatment with sodium bisulphite in the case of the wool yarns). In addition, in relation to natural yarns, setting cannot completely eliminate the residual torque of the singles yarns, and may also cause damage to the yarns.
Compared with permanent processing, physical torque balancing is a purely mechanical processing technique. The main point of the method is to fully utilize the structure of the yarns to balance the residual torque generated in different yarns while maintaining the elastic deformation characteristic of the yarns. Currently in the industry, separate machines are required to achieve torque balancing of the yarns, hence the cost is higher. The method comprises plying two identical singles yarns with a twist equal in magnitude but in the opposite direction or feeding two singles yarns with twist of the same magnitude but in opposite direction into the same feeder.
Recently, some new torque balancing methods for yarns also emerged in the Textile Research Journal, Vol. 65, No. 9, 1995, Sawhney and Kimmel described a series spinning system for processing torque-free yarns. The inner core of said yarns is formed by processing with an airjet system while outside the core is enwrapped with crust fibers similar to DREF-III yarns. In the Textile Research Journal, Vol. 62, No. 1, 1992, Sawhey etc. have suggested a method of processing ring cotton crust/polyester inner core yarns. Said yarns accomplish balancing by utilizing core yarns with opposite twisting direction from synthetic yarns, or applying heat processing on the polyester portion of said yarns. However, it is readily seen that the machines and processing techniques related to the aforesaid method are generally more complicated.
In the Textile Research Journal, Vol, 57, No. 10, 1997, Tao has processed the layer structure of the inner core-crust of rotor yarns to generate torque-free singles yarns, but said technique is not suitable for ring yarns.
In addition, U.S. Pat. No. 6,860,095 B2, filed by Tao et al. discloses a method of producing torque-free singles ring yarns. According to this patent application, a draft fiber is divided into a plurality of sub-assemblies of fibers. Each sub-assembly of fibers first attains an individual twist value during a false twisting, and then are twisted together to form the final yarns. The false twisting is controlled such that balancing of the internal torque of the final yarns is achieved. Furthermore, U.S. Pat. No. 7,096,655 B2 filed by Tao et al. discloses a method and apparatus for producing a singles ring yarn. In this method, a false twist device rotates at a first speed for twisting the fibers. Immediately after the first twisting step, a joint twist of the second twist in the same direction as the first twist and a third twist in a reversed direction is supplied to the preliminary yarn for producing final singles ring yarn. Moreover, the ratio of the first speed to the second speed is controlled for controlling the residual torque in the final singles ring yarn.
The aforementioned patents present the method and apparatus for singles ring yarn. However, the abovementioned patent application is more appropriate for torque-free singles ring yarn production in the laboratory scale. The yarn piecing-up and doffing process are not completely able to meet the practical requirements of large scale production in the textile industry. Furthermore, the spinning end-breakage when using ordinary cotton and the cost of investment and maintenance need to be further reduced for wide adoption in commercial application. In order to overcome the above shortcomings, two twisting points, instead of one twisting point, are adopted for the yarn false twisting to obtain the high false twist efficiency in this invention. In addition, the ratio of the velocity of the belt to the delivery speed of the yarn is controlled and the wrapping angle of the yarn on the belts is adjusted in order to obtain the desired properties of the final singles ring yarn.